Background: Medicare Part D (PD) provides prescription drug coverage through government-contracted private plans. It includes deductible, initial coverage, coverage gap, and catastrophic coverage (CC) phases. During the study period, Medicare reinsured 80% of drug costs above the out-of-pocket threshold (OOPT) in the CC, while also subsidizing costs for low-income beneficiaries with the Low-Income Cost Subsidy (LICS) in other phases. The Oncology Care Model (OCM), Medicare's value-based care model, considers reinsurance payments and LICS for PD drugs when benchmarking and evaluating performance. Methods: Using PD prescription data for 15 practices in The US Oncology Network, we obtained the gross drug costs below (GDCB) and above (GDCA) the OOPT, and LICS for prescriptions over a 6-year period (July 2016 – June 2022). We then evaluated the trends and patterns of per episode Part D expenditures (PEPDE) and LICS. Results: The analysis of 5.12M prescriptions for 99,623 patients revealed that episodes initiating in February and March had the highest PEPDE. However, per episode benchmark prices (PEBP) remained steady throughout. The seasonal variation in PEPDE was due to the PD plan design, with episodes crossing plan years (in January) having the lowest PEPDE. Consequently, performance periods (PP) starting with the calendar year had 15% higher PEPDE. Only 18% of patients received the LICS benefit, with 25% of payments occurring in January, coinciding with the entry of patients into CC. Conclusions: The design of PD plans creates seasonal patterns in PEPDE within models like OCM. Episodes with similar treatment patterns and beneficiary profiles can have different costs based on the start date, but a similar PEBP, leading to inconsistent performance outcomes. To ensure fair evaluation, a risk adjustment method that considers seasonality in the prediction model or balances fluctuations in expenses is needed. The impact of PD changes with the Inflation Reduction Act remains unexplored.